Protocol: Support for HTTP trailers

It doesn’t seem like the current ASGI specification allows applications to send HTTP trailers in their response, since it is indicated that servers should ignore any further messages on a channel once mode_body becomes false in a http.response.body message. This prevents application that stream their content from sending additionnal response information as trailers when it may not be known before the body has begun transfering, regarding things like caching, timing (Server-Timing), integrity check (Content-MD5, obsolete), digital signature (Signature, draft), etc.

WSGI doesn’t support it and I’m reading that overall support for them in various user agents tends to be flaky most of the time, but I couldn’t find any information regarding the omission of trailers from ASGI (whether deliberate or not) and I would like to know if there are any plans to add them at some point in the future, perhaps as an extension.

[andrewgodwin]

Yes, the use of them by GRPC is probably enough of a reason at this point - trust those folks to go resurrect a dusty old part of the HTTP spec 😃

[Kludex]

@andrewgodwin Given your last comment, should we reopen this issue?